This invention relates to a process and installation for recovering raw materials from a stream of residual or collected material which includes a plurality of different materials and which results from the manufacture of paper in a paper mill, from a material processing installation and/or a paper machine. In particular, to a process for recovering raw materials from a stream of residual or collected material via a screening operation and for separating an ash fraction containing black particles.
The manufacture of paper mill is a paper mill is usually effected in a manner such that a paper raw material suitable for the paper manufacture concerned is produced from fresh or waster paper fibers or from a mixture of said fibers in a material processing installation of the paper mill, which material processing installation is disposed upstream of the paper machine. Depending on the quality of the final product, this paper raw material consists of a mixture of fibers of very different qualities. Residual or collected material arises in the process water stream, both in the material processing installation during the material processing stage, and in the paper machine. Amongst other constituents, this residual or collected material contains coarse contaminants such as plastics and metal parts, fibrous material, black particles such as soot and printing ink residues, and contains fillers comprising a large proportion of kaolin, pigments, calcium carbonate and titanium dioxide. The process water stream which contains the residual or collected material snad which is discharged by the paper mill, and which is hereinafter termed the stream of residual or collected material, has hitherto usually been fed to a water treatment stage. The filtrate thereby obtained was then recycled to the paper mill, whilst the sludge removed from the water treatment stage had to be disposed of. The present invention is exclusively concerned with the stream of residual or collected material discharged from the paper mill.
In the process which is known from German Patent DE 40 34 054 C1, the entire residual or collected material is fed as a waste water stream to a residual waste water clarification installation and is subjected to a sedimentation operation, optionally with the assistance of flocculants. A thin sludge is then taken off from the sludge collecting compartment of the clarification installation. Residual waster water is admixed with the thin sludge in order to obtain an accurately determined consistency. The coarse contaminants contained in the stream of residual or collected material are subsequently removed by screening. The screened throughput stream is fed to a centrifuging installation having a plurality of hydrocyclones for separating black particles. The material stream which is discharged from the hydrocyclones is then fed to a screening stage in which fractions comprising fibers and agglomerates, as well as pigments and fillers, are produced. The pigments and fillers are separated from each other in a subsequent process stage. Finally, the fibers, pigments and fillers are recycled to the material processing stage of the paper mill.
Thus known process has various significant disadvantages. The clarification installation and the centrifuging installation with is hydrocyclones are very expensive and cost-intensive. Moreover, the energy requirement for operating the clarification installation, and particularly for operating the centrifuging installation, is very high, since there is a comparatively high pressure drop in the hydrocyclones during operation. Furthermore, the accurate adjustment of the thin sludge stream fed to the hydrocyclones is firstly expensive and secondly essential, since if the thin sludge concentration departs from the optimum consistency the hydrocyclones can no longer be operated or can only be operated at low efficiency. This known installation is explicitly designed for use in the wood pulp processing industry, since the fibers obtained from the waste water sludge are essentially pulp fibers. This known installation is not designed for use in an industry which also processes waste paper, where the fibre fraction obtained from the residual or collected material contains very many short and broken fibers, which makes the use of this fibre fraction very difficult and makes it impossible for most paper mills.